This disclosure relates to communication systems, and at least some of the examples disclosed herein relate more particularly to systems and methods for determining the number of channel estimation symbols to be transmitted to estimate a channel of a communications link based on the channel coherence bandwidth of the channel.
FIG. 1 is a diagram that illustrates an example of a wireless environment. An area 100 includes various transmitting and receiving devices 102, 104, and 106. These devices 102, 104, and 106 can include mobile phones, radio and television transmitters, wireless networking devices, etc. Some of the devices 102, 104, and 106 are mobile devices; some are not mobile. Mobile or not, however, the communication environment in which these devices operate is constantly changing. Signals from these devices 102, 104 and 106 reflect off buildings 108, vehicles 110 and 112, hills 114 and other features of the geographic area 100. Further, features of the area 100 are changing. Vehicles 110 and 112 move, people move within the area 100, weather patterns change, new buildings are built, etc. All of these and many other factors lead to a constantly changing communications environment.
The characteristics of wired communications channels, on the other hand, tend to be more consistent, even though they may vary with temperature, equipment changes, etc. Because of this relative consistency, it can be advantageous to estimate certain channel characteristics in ways not used in a wireless communication system, even if these wired systems use similar modulation techniques.
One example of a wired system is the system defined by the Multimedia over Coax Alliance (MoCA™). In a MoCA system, coaxial cables are used to connect components of the network, such as computers, TVs, set top boxes and radios, and generally, to distribute Ethernet signals throughout a home or building. MoCA systems are generally used to allow such entertainment devices within a home network to communicate with one another and share data, including multimedia data, such as television shows, movies, internet data, music, video clips, etc. One advantage of such MoCA systems is that new home wiring might be avoided because many homes already have adequate coaxial wiring installed. MoCA systems are typically used to distribute high-quality multimedia content and high-speed data with throughput exceeding 100 megabit per second. MoCA devices generally communicate with one another in the 1 GHz microwave band using orthogonal frequency-division multiplexing (OFDM) modulation. The OFDM modulated signals used by MoCA are communicated over MoCA channels using frequency-division multiplexing (FDM). In MoCA systems that use OFDM, each MoCA channel is formed from one of a large number of closely-spaced orthogonal sub-carriers. These MoCA channels are typically used to carry data. Each sub-carrier is typically modulated with a conventional modulation scheme at a low symbol rate, maintaining total data rates similar to conventional single-carrier modulation schemes in the same bandwidth. Some example modulations include quadrature amplitude modulation (QAM) or phase shift keying (PSK) modulation.
In order to take advantage of the maximum bandwidth of each channel, some systems may characterize each channel between each device and each other device. The characteristics of each channel are determined by transmitting an error vector magnitude (EVM) probe consisting of a fixed pattern from one device, i.e., a node in the network, to each other device that serves as a node on the network. Each such receiving device measures the deviation from the fixed pattern of the EVM probe in order to determine the amount of distortion to symbols transmitted over the channel due to gain, phase, delay and other characteristics of the channel.
In some multi-carrier communication systems, such as OFDM systems, channel estimation is used to characterize each channel so that the effects of variations in gain, phase and delay can be removed from the playload in order to provide reliable decoding of data in a communication system. Wireless communications will often experience different channel responses either in different environments or at different times, or both. These changes may be due to multi-path phenomena, for example. Channel estimation can be used to improve the quality of communications in a communication environment. Wired systems may also benefit from channel estimation because the channel estimation information can be used to characterize the wired communication environment. This environment may also vary over time due to component changes, temperature changes, etc.
Some OFDM systems may use a pilot sub-carrier to provide channel estimation with respect to data sub-carriers. In OFDM systems there are generally enough pilot sub-carriers and the distribution of the pilot sub-carriers may be uniform and contiguous such that the channel response of the data sub-carriers can be estimated relatively accurately from a measured pilot sub-carrier channel response. Accordingly, Channel estimation may be performed using OFDM symbols where some subset of subcarriers is used for channel estimation and the remaining subset of subcarriers is used for data. The subset of subcarriers used for channel estimation may change from OFDM symbol to OFDM symbol. OFDM symbols which consist solely of channel estimation subcarriers are called channel estimation symbols. In some OFDM systems such as MOCA systems, a complete OFDM symbol (a symbol that is transmitted on all sub-carriers) is transmitted to estimate the channel. Such symbols are called channel estimation symbols. Some protocols, such as MoCA 1.0, mandate that every packet start with exactly two channel estimation symbols. Since channel estimation symbols take up bandwidth, there is a need for a method and apparatus that can reduce the number of channel estimation symbols and thus increase the data throughput over the network.